Wash. Rev. Code § 85.05.220
In case it shall be found necessary to enlarge or strengthen the system of dikes sought to be connected with, by reason of such connection, the jury shall determine the cost of such enlarging or strengthening, and said petitioner district shall have the right, by and through its representatives, assistants and employees, to make such improvement on the system of such other district as may have been found necessary upon the hearing of said petition, and the costs thereof shall be assessed against the landowners of said petitioner district to be benefited by the construction of said entire system, and no additional cost or burden, by reason of such improvement, shall be thrown upon the landowners of said district sought to be connected with.
RCW 85.05.220